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The B777 fuel totalliser!!

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The B777 fuel totalliser!!

Old 16th Apr 2009, 18:19
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Exclamation The B777 fuel totalliser!!

Hey all.
One of my buddies had an issue on a 777 some 2-3 nights ago.Was flying in from a major city into another.A/C fuelled with some 110T fuel.Ground engineer gives the captain the fuel log which has fuel uplifted and fuel that should be on board.Along with specific gravity and all that info.All is ok,right until the captain(my friend) checks the fuel onboard with what should have been onboard and it differs by just over 0.5T.
One thing led to another and the AME managed to convince the captain that the fuel onboard was actually what he had pumped in and the the totalliser was underreading!
A/C was dispatched and during cruize and descent and subsequent landing the fuel figures were as a matter of fact exactly as they should have been thru the flight.
Can anyone pls throw some light on this.What kind of a totalliser do u have on the 777.And what are the chances of it actually under/over reading.I thought that this issue had been resolved years ago with the use of some very modern equipment on the 777 fuel systems.Pls help here.
The "F/O" on that day just happened to be a senior instructor doing this captain's line check!! Therefore this query.....
Thanks in advance.....

Last edited by King on a Wing; 16th Apr 2009 at 18:25. Reason: Typo...
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Old 16th Apr 2009, 21:40
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Dunno about the 777, but a certain amount of difference between fuel summation unit and uploaded fuel is permissible. On our 737 up to 300kg, everything above that required a dripstick reading which in itself is a messy and failure prone process. Now, we cannot load nearly as much fuel as a 777, so i would expect quite bit more margin on the big beast.
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Old 16th Apr 2009, 23:05
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The B777 fuel totalliser!!

King on a wing

Fuel Quantity and totalizing has not changed much for the last couple of decades. Totalizing will allways equal the sum of calculated fuel in each tank that are computed seperately. The components that are quite redundant will read fuel quantity with varied levels of accuracy at different levels as thes components are subject to age, wear and corrosion. Contaminants take into concideration as well.

Having said that the level of total quantity accuracy will allways be a variable that is impossible to maintain as the fuel level changes. Other variables are APU fuel burn with respect to doccumented gate fuel and specific gravity variance.
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Old 17th Apr 2009, 00:16
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Snoop

Fuel measurement is ultrasonic with something in the region of 20 probes per tank (depending on a/c model number) with ultrasonic densitometers also bought into the equation. As previously posted there is some 'leeway' in the difference between actuals and uplift calcs. Also there will be a certain amount of aeration of the fuel after uplift and the true indication can sometimes take a while to settle down.
Failing that it could be water....
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Old 17th Apr 2009, 00:26
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On our 737's as long as the calculated and indicated readings are within 3%, you're good to go. Of course, we don't get anywhere near 110 t of fuel!. But there would have to be some tolerance that is applied.
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Old 17th Apr 2009, 01:48
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110T is probably below full tanks however when departing at Maximum Brake Release Weight, total ‘Departure Fuel’ can exceed the ‘Total Required’. This could be caused by gauge error and not necessarily over-fuelling.

If one or more wing tanks shut off above the ‘Distribution Required'
use the ‘Distribution Required’ value, in lieu of the tank gauge reading for that tank, provided the quantity is checked at the appropriate SG.

Consider using ‘Total Prior To Fuelling’ plus ‘Tanker Uplift’ as the total fuel for loading purposes
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Old 17th Apr 2009, 13:57
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777s have ultrasonic fuel qty indication system. As mentioned above there are some 20 probes in each tank (60 total) to detect qty. Few clarifications regards to above posts:

1) there is a Water detector in both tanks and if there is Water then the Maintenance Page will def show the message.

2) Was the APU running all along the transit???? and how long was the transit for?? If it was.....then it does consume a significant amout and causes lot of discrepancies.

3) the density of the uplifted fuel is another factor. What the refueller mentions might not be exactly accurate untill and unless he or she has actually checked it just before refuelling. If the uplift is quite large then theres lot of room for discrepancies.

4) Going back the fuel system on 777s.....the whole logic set-up in the FQPU computer relies on these 60 probes in the tanks ( 20 in each). Now if there is some kind of problem in the sensing of these probes then the FQPU starts showing the quantities according to the Logic status of the software.....meaning the average qty coming from the serviceable probes and then gets more complicated when the quantities start to go blank when the minimum number required to detect actual quantity goes u/s.The ones which really cause trouble is those which are intermittent.....meaning they are serviceable 1 minute and goes u/s in second minute......the computer monitors them constantly and goes mental with the calculations if it has a good number of probes which are working 1 second then not working and then start working again.......

There are lots of things that could have caused those errors.....pin-pointing it to something that actually was the cause is really difficult......anyways the fuel was enough to land safely and thats what matters most!!!!
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Old 17th Apr 2009, 16:56
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Bit of a strange story. There are two elements here.
If the Capt requested 110T. and the totaliser reads 109.5, then I would add more fuel to get the totaliser correct.
The B777 totaliser is just a sum of the three tanks.
Trying to convince the Capt that it is reading wrong means identifying which tank is at fault, doing a stick check, and raising an MEL item. Much too much work! On our airline the allowed uplift discrepancy is 2000kg for a B777.
I refuel B777 every day, but not 110 tons, ours only fly for 11 hours and 85 tons is enough. It is very rare to get a gauging problem, but at the end the difference between calculated uplift and actual uplift is in the region of 300 kgs. When you are uplifting 100000 litres a small error in the SG can make a difference.
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Old 18th Apr 2009, 07:42
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Yes my friends,
I had an opportunity to speak to some of my more errm 'techically qualified' colleagues.As in AME's on the line who work in major maintenence on the system.And Mr Fixer seems to have nailed it spot on.This airplane has some 20 sensors per tank.Each of these is effectively a "radio doppler".And works on the same principle.Calculates the height of fluid by measuring the difference in the speed of the sonic signal sent out thru one sensor to another,as against the ACTUAL local speed of sound(which is a preset value) thru standard temp air.This difference is then converted very accurately into height in the tank in mm.Which translates into quantity of fuel AT THAT sensor given the actual specific gravity of fuel at that point........(Whew.....!!!)
Now that's what I understood.....
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Old 18th Apr 2009, 11:55
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Well done King!!! now you understood a bit of it!!! thats only the basics of this a/c fuel system. 777s look so very easy to fly and maintain but when we scratch the surface, its far more complex, but nowhere near to the Airbuses!!!! here goes the eternal debate of Boeing vs Airbus!!!
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Old 19th Apr 2009, 02:28
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B777 fuel totalliser

The Captain could check the uplift density of fuel on Fuel qty maintenance page.
777 Fuel syonptics display gives only the quantity of fuel,it does not indicate the uplift density.He could have compared the uplift density indicated in the fuel log book with that indicated on Aircraft Fuel qty maintenance page. If they differed he could indicate on fuel log and can make a tech log entry , so that the problem can be fixed.Many Airlines allow certain amount of difference between fuel truck gauges and aircraft fuel gauges.The source of data for display at Refuel station gauges at the wing and fuel synoptics is same i.e FQPU(Fuel quantity processor Unit).The refuel station gauges get the data on ARINC 429 bus and Fuel synoptics gets data from PDS which gets data on 629 bus from FQPU.
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Old 19th Apr 2009, 07:01
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One thing led to another and the AME managed to convince the captain that the fuel onboard was actually what he had pumped in and the the totalliser was underreading!
Captain's choice, I'd say. You could also claim that the fuelling truck's gauges were overreading ... or someone entered the arrival fuel incorrectly.

However, just because there are 20 high tech ultrasonic sensors in each tank, it doesn't mean that they are infallible. If they are anything like the ultrasonic potable water level detectors on our 744ER's, the tanks could be reading full when they are empty : P

The 767 totaliser system is more accurate at certain levels, but the 777 system may have an advantage with the number of sensors in each tank. The wing being such an irregular shape, it is always difficult to calculate the volume in these tanks.

Rgds
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Old 25th Apr 2009, 12:32
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Wing Shape Irrelevent

The loads talked about in this post (85T and 105T) would both result in full wing tanks, so the talk about funny shaped wings is irrelevent.

The wings hold about 10,000 Gals eash (approx 30T depending on density).

The fuel probes (in any technology) actually give a height of fuel above the probe. This can then be converted into a volume, and then the actual measured density will be used to convert it into a mass. This is all fairly straight-forward when all is working, but if one probe is mis-reading it starts to get complicated. Computers are not very good at looking at a set of data and saying "that reading is silly, let's ignore it"

The bowser will also give a volume reading, which is converted using the fuel farm density to give a mass. I guess if the bowser is parked out in the sun fully loaded for an hour, the temperature of the fuel could go up, and hence the density of the uploaded fuel will be lower than that reported at the fuel farm.
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